Growth rates, photosynthetic responses and the activity, amount and CO2 affinity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) were determined for common marine macroalgae grown in seawater (containing 14.5 ± 2.1 µM CO2) or CO2-enriched seawater (averaging 52.8 ± 19.2 µM CO2). The algae were grown in 40 L fiberglass tanks (outdoor) for 4–15 weeks and in a field experimental setup for 5 days. Growth rates of the species studied (representing the three major divisions, i.e. Chlorophyta, Rhodophyta and Phaeophyta) were generally not significantly affected by the increased CO2 concentrations in the seawater medium. Rubisco characteristics of algae cultivated in CO2-enriched seawater were similar to those of algae grown in nonenriched seawater. The lack of response of photosynthetic traits in these aquatic plants is likely to be because of the presence of CO2 concentrating mechanisms (CCMs) which rely on HCO3– utilization, the inorganic carbon (Ci) form that dominates the total Ci pool available in seawater. Significant changes on the productivity of these particular marine algae species would not be anticipated when facing future increasing atmospheric CO2 levels.